Nióbias sulfatadas e fosfatadas como catalisadores ácidos na esterificação de ácido acético com etanol visando o beneficiamento de bio-óleo

Abstract

In the last decades, several studies have been developed owing to fuels production from renewable sources. Among the most studied biofuels appears bio-oil, a mixture of organic compounds rich in oxygenates. After being produced, crude bio-oil cannot be used as a fuel for some reasons, among which can be highlighted its high acidity due to the presence of carboxylic acids. So, it is necessary to carry out beneficiation processes to fit bio-oil as a fuel, such as, the esterification of carboxylic acids with alcohols over acid catalysts, which results in esters production and acidity reduction. The replacement of the usual liquid acid catalyst with solid ones is environmentally and economically favorable. In this work, solid acid catalysts based on niobium were synthesized for esterification of acetic acid aiming at bio-oil upgrading. Sulphated and phosphated niobias were synthesized by treating hydrated niobium oxide with different sulfuric acid and phosphoric acid solutions (0.5, 1 and 2 M). The ratio between solid and solution was also varied (20: 300 and 20: 500) and the treatment time was maintained at 1h. After treatment, the suspension was filtered and the solid dried and calcined at 500 oC. Therefore, 12 catalysts were obtained, 6 of them were sulphated and 6 were phosphated, besides niobium phosphate and niobium oxide calcined at the same condition. The catalytic activities of these samples were evaluated in the model reaction of esterification of acetic acid with ethanol, at 60 °C and 1 atm. The materials were also characterized by the following techniques: X-ray diffraction; Infrared absorption spectroscopy of samples diluted in KBr and also adsorbed in pyridine; Thermoprogrammed desorption (TPD) of adsorbed ammonia, Textural analysis with N2 physisorption and Elementary analysis. By elementary analysis, it was possible to confirm the sulphate and phosphate ions incorporation into niobium oxide. From TPD, it was possible to observe the increase in the surface acidity of the solids after the treatment of niobium oxide, due to the higher amount of NH3 adsorption. After the treatment, all catalysts achieved higher surface areas and smaller volumes and pore diameters compared to niobium oxide. From XRD technique, it was observed that all sulphated solids are less crystalline than pure niobium oxide, and in phosphate ones the treatment caused amorphization. The FTIR confirmed the presence of S=O and P=O species on the treated samples. DRIFTS identified the presence of Lewis and Brönsted acid sites. The sulphated and phosphated catalysts obtained in the 2 M concentration and 20:300 ratio as well as bulk niobium oxide and niobium phosphate were all tested for reuse. Niobium oxide and niobium phosphate showed similar activities to those of the respective fresh catalysts, even after the third use. However, functionalized niobias showed severe decrease in activity.